Vapor deposition of rhenium and rhenium-tungsten alloy coatings

ABSTRACT

A PROCESS FOR COATING A SUBSTRATE WITH RHENIUM. A MIXTURE OF GASEOUS HYDROGEN AND A RHENIUM OXYCHLORIDE VAPORIZED IN AN INERT GAS IS PASSED OVER THE SUBSTRATE, WHICH IS HEATED TOA TEMPERATURE EFFECTING REDUCTION OF THE OXYCHLORIDE AND DEPOSITION OF RHENIUM METAL ON THE SUBSTRATE. A RHENIUM-TUNGSTEN ALLOY COATING CAN BE DEPOSITED BY INCORPORATING TUNGSTEN HEXAFLUORIDE IN THE GAS MIXTURE PASSED OVER THE SUBSTRATE.

NOV. 21, 1972 RE'NNE'R ETAL 3303305 VAPOR DEPOSITION OF RHENIUM AND RHENIUM-TUNGSTEN ALLOY COATINGS Filed Oct. 2'7, 1970 INVENTORS.

I Abner Brenner BY Harvey J. Anderson ATTORNEY.

United States Patent US. Cl. 117107.2 1 Claim ABSTRACT OF THE DISCLOSURE A process for coating a substrate with rhenium. A mixture of gaseous hydrogen and a rhenium oxychloride vaporized in an inert gas is passed over the substrate, which is heated to a temperature efiecting reduction of the oxychloride and deposition of rhenium metal on the substrate. A rhenium-tungsten alloy coating can be deposited by incorporating tungsten hexafluoride in the gas mixture passed over the substrate.

BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission.

Rhenium in the form of pure, coherent coatings of appreciable thickness would be useful in many applications, particularly those requiring resistance to high temperatures. Rhenium has a higher melting point (3180 C.) than any of the metals except tungsten. Compared to tungsten, rhenium offers various advantages: for example, it is ductile at room temperature, it is not subject to the formation of carbides, it is more resistant to attack by water vapor at elevated temperatures, and it is more resistant to salt spray and to exposure to the atmosphere.

Electrodeposition is not a very satisfactory method of producing rhenium coatings, since the cathode efliciency is low and the resulting coatings are sound only in thicknesses of several microns. Also, electrodeposited coatings quickly tarnish or corrode in the atmosphere.

Chemical vapor deposition (CVD) is known to be generally superior to electrodeposition as a method of forming rhenium coatings, but none of the CVD processes has been found entirely satisfactory. In one type of CVD process, rhenium coatings have been produced by passing a vaporized rhenium compound over a substrate heated to a temperature sufiicient to thermally decompose the compound, but thermal decomposition requires the use of undesirably high temperatures-e.g., 1000-2000 C.- and sometimes the use of very low pressures. In another type of CVD process, rhenium coatings have been deposited on a heated substrate by contacting the substrate with a gaseous mixture of a rhenium compound and a reducing agent therefor. For instance, the reduction of rhenium hexafluoride with hydrogen has received much attention, but in that process part of the rhenium deposits a comparatively low-temperature process for the deposition of rhenium and rhenium-alloy coatings.

It is another object to provide a low-temperature process for the deposition of smooth and bright rhenium coatings, said process being characterized by the deposition of little or no rhenium in the form of loose powder.

Other objects of this invention will become apparent in the course of the following description.

BRIEF DESCRIPTION OF THE DRAWING The single figure is a schematic diagram of a reactor system in which rhenium-containing coatings can be produced in accordance with this invention.

PREFERRED EMBODIMENTS OF THE INVENTION In the preferred form of our process as directed to the formation of rhenium metal coatings, streams of gaseous hydrogen and rhenium oxychloride vaporized into argon carrier gas are introduced continuously to a reaction zone free from oxidizing gases. The two streams are mixed thoroughly and passed over a substrate maintained at a temperature promoting reduction of the oxychloride and the deposition on the substrate of an adherent coating of rhenium. Gaseous products from the reduction reaction are removed continuously from the reaction zone.

Our process can be conducted in simple apparatus, such as that illustrated in the accompanying figure. As shown, a cylindrical glass reaction chamber 1 is mounted within a larger glass cylinder 2 to form an annulus 3 therewith. Centrally mounted within the chamber 1 is a metal tube 4, or substrate, containing a thermocouple connected to an external temperature indicator (not shown). Mounted within the annulus 3 is an induction coil 5 for heating the substrate. As indicated, the annulus 3 is filled with a heat transfer liquid 6 whose temperature is maintained at a selected constant value by a thermostatically controlled immersion heater 7. Mounted within the lower portion of the reaction chamber is a rotatable propeller-and-permanent-magnet assembly 8, driven by a rotatable external magnet (not shown). The upper end of the reaction chamber is closed by a tubular glass stopper 9, which has been evacuated to reduce heat transfer. As indicated, a vent line 10 is connected into the upper end of the chamber and extends through the stopper 9. Connected into the lower portion of the reaction chamber are externally extending lines 11 and 12. An external part of line 11 is formed with a U-bend 13, which contains a quantity of solid or liquid rhenium oxychloride and is immersed in a thermostatically controlled heating bath 14. Lines 11 and 12 are respectively connected to supplies of argon and hydrogen (not shown).

In a typical use of the apparatus shown, the reaction chamber and its associated lines are evacuated and then filled with inert gas. The electrical input to the induction coil 5 is adjusted to a value maintaining the substrate at the temperature desired for deposition of the rhenium coating. The temperature of the heat transfer liquid 6 is adjusted to a value minimizing both condensation of the oxychloride and deposition of rhenium on the wall of the reaction chamber. The U-tube 13 is maintained at a temperature somewhat below the boiling point of the oxychloride, and the rate of evaporation of the latter is controlled by adjustment of the argon flow rate. The resulting argon-oxychloride mixture sweeps into the lower part of the reaction chamber 1, where it is mixed with incoming hydrogen b the stirrer 8. The stirred mixture flows past the substrate 4, and those gases reaching the upper part of the reaction chamber exit through line 10.

The following examples illustrate our process as conducted in the above-described apparatus.

3 EXAMPLE I In this run the U-tube 13 was loaded with a 0.82-gram sample of perrhenyl chloride (ReO Cl), prepared by passing hydrogen chloride over rhenium heptaoxide. The U-tube was maintained at a temperature of 75 C. throughout the run (65 minutes). Argon was passed over the sample at a rate of 40 s.cc./min. to carry the oxychloride vapor into the reaction chamber (volume, 500 cc.). The hydrogen flow through line 12 was maintained at 110 s.cc./min., providing a hydrogen-to-oxychloride mol ratio of approximately 100. The heat transfer liquid 6 (a mineral oil) was maintained at 120 C., and the substrate, which was a gold-plated nickel tube with an outside diameter of 14; inch, was held at a temperature of 350 C.

The resulting rhenium coating on the substrate was approximately 0.7 mil thick. The deposit was smooth and bright, but was under stress. The amount of rhenium deposited as a powder was small compared to that produced in experiments where rhenium hexafiuoride was reduced with hydrogen, in the same apparatus. The efficiency of metal deposition was about 60%.

Subsequent experiments established that the rhenium coating on the substrate tended to crack and exfoliate if deposited in thicknesses exceeding about one mil. It was found, however, that adhesion was improved if the substrate was provided with a thin coating of electrodeposited rhodium (e.g., a coating one or two microns thick) prior to exposure to the reactant gases.

Reduction to the metal was found to take place at temperatures at least as low as 300 C. At 450 C., the deposited rhenium coating was duller but also exhibited less cracks. At 600 C., the coating was granular or powdery. Similar results were obtained when substrates composed of nickel or stainless steel were employed.

EXAMPLE II In this run the U-tube contained a sample of rhenium oxytetrachloride (ReOCl prepared by heating rhenium heptaoxide with thionyl chloride and distilling off the excess of the latter. Throughout the run (45 minutes), the U-tube was maintained at 170 C. The argon flow rate was 80 s.cc./min., and the hydrogen flow rate was 400 s.cc./min., providing a hydrogen-to-oxychloride mol ratio of 300. The heat transfer liquid (a commercial polyphenyl ether) and the substrate (a stainless steel tube) were maintained at 220 C. and 700 C., respectively.

The resulting rhenium coating on the substrate was 0.3 mil thick. The deposit was smooth and bright but stressed. Subsequent experiments established that reduction to the metal takes place at temperatures at least as low as 350 C. At 600 C. to 700 C., the efiiciency of deposition was found to be higher (-32% but the coating was duller and much of the rhenium deposited as a powder on the walls of the reaction chamber.

Referring now to the production of rhenium-tungsten alloy coatings, we have found that the co-deposition of tungsten represses the tendency of rhenium to form cracked or exfoliated deposits. The following is an example of this form of our process.

EXAMPLE III The above-described apparatus was modified to permit the introduction of gaseous tungsten hexafluoride to the line conveying argon to the U-tube. Using this arrangement, a gas mixture consisting of 86 mol percent hydrogen, 13 mol percent tungsten hexafluoride, and 1 mol percent perrhenyl chloride was passed over a tubular stainless steel substrate maintained at approximately 400 C. The heat transfer liquid (a mineral oil) was maintained at 130 C. After 30 minutes, the run was terminated and the coated substrate was immersed in concentrated HCl to remove the substrate. Removal of the substrate left a smooth, crack-free tube of tungsten-6% rhenium alloy. The tube had a wall thickness of 2.0 mils.

Although our process can be conducted conveniently in the apparatus shown in the figure, it will be understood that it can be conducted in other apparatus. For example, the reactant gases can be passed through a heated tube to deposit the desired coating thereon.

What is claimed is:

1. The process of forming on a substrate a rhenium coating having a thickness exceeding about one mil comprising providing said substrate with a thin coating of rhodium, heating the rhodium-coated substrate to a temperature in the range of about 250 C. to 600 C., and contacting the heated substrate with a gaseous mixture of hydrogen and a rhenium oxychloride.

References Cited UNITED STATES PATENTS 3,343,979 9/1967 Hamrin, Jr 1l7--107.2 R 1,877,261 9/ 1932 Weiger -134 V 3,374,092 3/ 1968 Marshall 75-434 V 3,140,941 7/1964 Walter 75--134 V OTHER REFERENCES Vapor Deposition, Powell et al., N.Y., Wiley & Sons, 1966, pp. 312-316.

ALFRED L. LEAVITT, Primary Examiner K. P. GLYNN, Assistant Examiner US. Cl. X.R.

29-194; 75-134 V; 1l7-71 M 

